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Name: isabelle
Status: educator
Age: 50s
Location: N/A
Country: N/A
Date: 2000


Question:
I am looking for a good explanation (for my grade six students)of the properties of thixotropic mixtures. I am thinking in particular of the wonderful goop made from white glue, water, and borax and also the "magic mud" mixture of cornstarch and water.

Why do they tend to be more liquid when they are left alone, and more solid when agitated?


Replies:
The explanation in the science book we use ("Innovations in Science" -- sorry I don't have it with me at the moment)seems to say that agitating the cornstarch and water mixture makes it more liquid because the molecules are excited to move faster.

However it is not our experience that agitation makes these mixtures more liquid. The goop we made from the glue and borax spreads over the table when left alone, but when worked in the hands can be formed into a bouncable ball.

I would appreciate any help!

The explanation you recall from the book explains why most liquids become thinner when they are heated. In the case of your gak and water/cornstarch mixtures, the situation is a little different. The important factor controlling the viscosity of these materials is that they are suspensions of polymeric molecules.

In the cornstarch case, the shear-thickening behavior arises from the interactions between the polymer chains. When the flow rate is low, the chains can easily slide past each other, so that the mixture is quite liquid. As the flow (actually shear) increases, the polymer chains get in each others' way as they move. This resists the movement, and the mixture becomes more viscous.

I don't know that gak is actually shear-thickening, but it does have different modes of motion at different timescales. On the fast time scale, it jiggles like jello and bounces like rubber, and on a slower time scale it flows like tar. (Silly putty does this, too, but I'm not sure what its molecular mechanism is.) The reason this happens is that the polymer chains in gak (from the glue) are actually chemically cross-linked by the borax. These cross-links make the entire sample one big interlocked network, which is why it is gelled. However, the chemical nature of these cross-links (borate esters) is such that they rapidly fall apart and re-form in water. So, over a longer time, the polymer chains can flow past each other.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois



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